Fifty years ago on the 20th of July at nearly 11 o’clock at night, my brothers and I were peering through the screen door of a very small 1960’s Shasta compact car trailer watching the TV set on the picnic table outside the trailer door. Our family was at a camp ground in southern Michigan and the mosquitos were fierce (hence why we were inside the trailer looking out through the screen). Neil Armstrong was about to be the first human to step foot on the Moon. The image on the TV was a fuzzy black and white, with barely recognizable shapes clouded even more by the dirt and dead bugs on the screen, but it is a memory etched in my mind. I was 10 years old and I was convinced that when I grew up I would visit the Moon myself, because by then Moon travel would be like flying to Europe. It didn’t turn out that way, and fifty years later it’s a struggle to even get back there.

The dangers could have become life-threatening for the crew of Apollo 11. If they miscalculated their trajectory home and had bounced off the Earth’s atmosphere, they would have become a tragic demonstration of the chaos of three-body orbits.

So maybe I won’t get to the Moon, but maybe my grandchildren will. And if they do, I hope they know something about the three-body problem in physics, because getting to and from the Moon isn’t as easy as it sounds. Apollo 11 faced real danger at several critical points on its flight plan, but all went perfectly (except overshooting their landing site and that last boulder field right before Armstrong landed). Some of those dangers became life-threatening for the crew of Apollo 13, and if they had miscalculated their trajectory home and had bounced off the Earth’s atmosphere, they would have become a tragic demonstration of the chaos of three-body orbits. In fact, their lifeless spaceship might have returned to the Moon and back to Earth over and over again, caught in an infinite chaotic web.

The complexities of trajectories in the three-body problem arise because there are too few constants of motion and too many degrees of freedom. To get an intuitive picture of how the trajectory behaves, it is best to start with a problem known as the restricted three-body problem.

The Saturn V Booster, perhaps the pinnacle of “muscle and grit” space exploration.

The Restricted Three-Body Problem

The restricted three-body problem was first considered by Leonhard Euler in 1762 (for a further discussion of the history of the three-body problem, see my Blog from July 5). For the special case of circular orbits of constant angular frequency, the motion of the third mass is described by the Lagrangian

where the potential is time dependent because of the motion of the two larger masses. Lagrange approached the problem by adopting a rotating reference frame in which the two larger masses m1 and m2 move along the stationary line defined by their centers. The new angle variable is theta-prime. The Lagrangian in the rotating frame is

where the effective potential is now time independent. The first term in the effective potential is
the Coriolis effect and the second is the centrifugal term. The dynamical flow in the plane is four
dimensional, and the four-dimensional flow is

where the position vectors are in the center-of-mass frame

relative to the positions of the Earth and Moon (x1 and x2) in the rotating frame in which they are at rest along the x-axis.

A single trajectory solved for this flow is shown in Fig. 1 for a tiny object passing back and forth chaotically between the Earth and the Moon. The object is considered to be massless, or at least so small it does not perturb the Earth-Moon system. The energy of the object was selected to allow it to pass over the potential barrier of the Lagrange-Point L1 between the Earth and the Moon. The object spends most of its time around the Earth, but now and then will get into a transfer orbit that brings it around the Moon. This would have been the fate of Apollo 11 if their last thruster burn had failed.

Fig. 1 The trajectory of a tiny object in the planar three-body problem interacting with a large mass (Earth on the left) and a small mass (Moon on the right). The energy of the trajectory allows it to pass back and forth chaotically between proximity to the Earth and proximity to the Moon. The time-duration of the simulation is approximately one decade. The envelope of the trajectories is called the “Hill region” named after one of the the first US astrophysicists George William Hill (1838-1914) who studied the 3-body problem of the Moon.

Contrast the orbit of Fig. 1 with the simple flight plan of Apollo 11 on the banner figure. The chaotic character of the three-body problem emerges for a “random” initial condition. You can play with different initial conditions in the following Python code to explore the properties of this dynamical problem. Note that in this simulation, the mass of the Moon was chosen about 8 times larger than in nature to exaggerate the effect of the Moon.

In the code, set the position and speed of the Apollo command module on lines 56-59 and put in the initial conditions on line 70. The mass of the Moon in nature is 1/81 of the mass of the Earth, which shrinks the L1 “bottleneck” to a much smaller region that you can explore to see what the fate of the Apollo missions could have been.

When Newton developed his theory of universal gravitation, the first problem he tackled was Kepler’s elliptical orbits of the planets around the sun, and he succeeded beyond compare. The second problem he tackled was of more practical importance than the tracks of distant planets, namely the path of the Earth’s own moon, and he was never satisfied.

Newton’s Principia and the Problem of
Longitude

Measuring the precise location of the moon at very exact times against the backdrop of the celestial sphere was a method for ships at sea to find their longitude. Yet the moon’s orbit around the Earth is irregular, and Newton recognized that because gravity was universal, every planet exerted a force on each other, and the moon was being tugged upon by the sun as well as by the Earth.

Newton’s attempt with the Moon was his last significant scientific endeavor

In Propositions 65 and 66 of Book 1
of the Principia, Newton applied his
new theory to attempt to pin down the moon’s trajectory, but was thwarted by
the complexity of the three bodies of the Earth-Moon-Sun system. For instance, the force of the sun on the
moon is greater than the force of the Earth on the moon, which raised the
question of why the moon continued to circle the Earth rather than being pulled
away to the sun. Newton correctly recognized that it was the Earth-moon system that was in orbit around the sun,
and hence the sun caused only a perturbation on the Moon’s orbit around the
Earth. However, because the Moon’s orbit
is approximately elliptical, the Sun’s pull on the Moon is not constant as it
swings around in its orbit, and Newton only succeeded in making estimates of
the perturbation.

Unsatisfied with his results in the Principia, Newton tried again, beginning
in the summer of 1694, but the problem was to too great even for him. In 1702 he published his research, as far as
he was able to take it, on the orbital trajectory of the Moon. He could pin down the motion to within 10 arc
minutes, but this was not accurate enough for reliable navigation, representing
an uncertainty of over 10 kilometers at sea—error enough to run aground at
night on unseen shoals. Newton’s attempt
with the Moon was his last significant scientific endeavor, and afterwards this
great scientist withdrew into administrative activities and other occult
interests that consumed his remaining time.

Race for the Moon

The importance of the Moon for navigation was too pressing to ignore, and in the 1740’s a heated competition to be the first to pin down the Moon’s motion developed among three of the leading mathematicians of the day—Leonhard Euler, Jean Le Rond D’Alembert and Alexis Clairaut—who began attacking the lunar problem and each other [1]. Euler in 1736 had published the first textbook on dynamics that used the calculus, and Clairaut had recently returned from Lapland with Maupertuis. D’Alembert, for his part, had placed dynamics on a firm physical foundation with his 1743 textbook. Euler was first to publish with a lunar table in 1746, but there remained problems in his theory that frustrated his attempt at attaining the required level of accuracy.

At nearly the same time Clairaut and
D’Alembert revisited Newton’s foiled lunar theory and found additional terms in
the perturbation expansion that Newton had neglected. They rushed to beat each other into print, but
Clairaut was distracted by a prize competition for the most accurate lunar
theory, announced by the Russian Academy of Sciences and refereed by Euler,
while D’Alembert ignored the competition, certain that Euler would rule in
favor of Clairaut. Clairaut won the
prize, but D’Alembert beat him into print.

The rivalry over the moon did not
end there. Clairaut continued to improve lunar tables by combining theory and
observation, while D’Alembert remained more purely theoretical. A growing animosity between Clairaut and
D’Alembert spilled out into the public eye and became a daily topic of
conversation in the Paris salons. The
difference in their approaches matched the difference in their personalities,
with the more flamboyant and pragmatic Clairaut disdaining the purist approach
and philosophy of D’Alembert. Clairaut
succeeded in publishing improved lunar theory and tables in 1752, followed by
Euler in 1753, while D’Alembert’s interests were drawn away towards his
activities for Diderot’s Encyclopedia.

The battle over the Moon in the late 1740’s was carried out on the battlefield of perturbation theory. To lowest order, the orbit of the Moon around the Earth is a Keplerian ellipse, and the effect of the Sun, though creating problems for the use of the Moon for navigation, produces only a small modification—a perturbation—of its overall motion. Within a decade or two, the accuracy of perturbation theory calculations, combined with empirical observations, had improved to the point that accurate lunar tables had sufficient accuracy to allow ships to locate their longitude to within a kilometer at sea. The most accurate tables were made by Tobias Mayer, who was awarded posthumously a prize of 3000 pounds by the British Parliament in 1763 for the determination of longitude at sea. Euler received 300 pounds for helping Mayer with his calculations. This was the same prize that was coveted by the famous clockmaker John Harrison and depicted so brilliantly in Dava Sobel’s Longitude (1995).

Lagrange Points

Several years later in 1772 Lagrange discovered an interesting special solution to the planar three-body problem with three massive points each executing an elliptic orbit around the center of mass of the system, but configured such that their positions always coincided with the vertices of an equilateral triangle [2]. He found a more important special solution in the restricted three-body problem that emerged when a massless third body was found to have two stable equilibrium points in the combined gravitational potentials of two massive bodies. These two stable equilibrium points are known as the L4 and L5 Lagrange points. Small objects can orbit these points, and in the Sun-Jupiter system these points are occupied by the Trojan asteroids. Similarly stable Lagrange points exist in the Earth-Moon system where space stations or satellites could be parked.

For the special case of circular orbits of constant angular frequency w, the motion of the third mass is described by the Lagrangian

where the potential is time dependent because of the motion of the two larger masses. Lagrange approached the problem by adopting a rotating reference frame in which the two larger masses m1 and m2 move along the stationary line defined by their centers. The Lagrangian in the rotating frame is

where the effective potential is now time independent. The first term in the effective potential is the Coriolis effect and the second is the centrifugal term.

Fig. Effective potential for the planar three-body problem and the five Lagrange points where the gradient of the effective potential equals zero. The Lagrange points are displayed on a horizontal cross section of the potential energy shown with equipotential lines. The large circle in the center is the Sun. The smaller circle on the right is a Jupiter-like planet. The points L1, L2 and L3 are each saddle-point equilibria positions and hence unstable. The points L4 and L5 are stable points that can collect small masses that orbit these Lagrange points.

The effective potential is shown in the figure for m3 = 10m2. There are five locations where the gradient of the effective potential equals zero. The point L1 is the equilibrium position between the two larger masses. The points L2 and L3 are at positions where the centrifugal force balances the gravitational attraction to the two larger masses. These are also the points that separate local orbits around a single mass from global orbits that orbit the two-body system. The last two Lagrange points at L4 and L5 are at one of the vertices of an equilateral triangle, with the other two vertices at the positions of the larger masses. The first three Lagrange points are saddle points. The last two are at maxima of the effective potential.

L1, lies between Earth and the sun at about 1 million miles from Earth. L1 gets an uninterrupted view of the sun, and is currently occupied by the Solar and Heliospheric Observatory (SOHO) and the Deep Space Climate Observatory. L2 also lies a million miles from Earth, but in the opposite direction of the sun. At this point, with the Earth, moon and sun behind it, a spacecraft can get a clear view of deep space. NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) is currently at this spot measuring the cosmic background radiation left over from the Big Bang. The James Webb Space Telescope will move into this region in 2021.